U.S. patent application number 13/089418 was filed with the patent office on 2011-10-27 for electromagnetic flow meter.
This patent application is currently assigned to YAMATAKE CORPORATION. Invention is credited to Taka Inoue, Ichirou Mitsutake.
Application Number | 20110264382 13/089418 |
Document ID | / |
Family ID | 44370588 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110264382 |
Kind Code |
A1 |
Inoue; Taka ; et
al. |
October 27, 2011 |
ELECTROMAGNETIC FLOW METER
Abstract
Gain switching is performed by a DC amplifying circuit. The DC
amplifying circuit is provided with individual gain generating
circuits and a gain selecting circuit, and saturation preventing
circuits are provided in earlier stages than the individual gain
generating circuits. The individual gain generating circuit
generates a gain G1, the individual gain generating circuit
generates a gain G2 (where G2>G1), and the individual gain
generating circuit generates a gain G3 (where G3>G2). The gain
selecting circuit selects, as a used gain generating circuit, one
of the individual gain generating circuits, and sends the output
thereof to an A/D converting circuit in a later stage. A gain
switching instruction is sent to the DC amplifying circuit, which
controls the enabling/disabling of the saturation preventing
operations of the saturation preventing circuits and the selecting
operation for the gain selecting circuit, to prevent the occurrence
of saturation in the individual gain generating circuits that
generate higher gains than the gain generated by the used gain
generating circuit.
Inventors: |
Inoue; Taka; (Tokyo, JP)
; Mitsutake; Ichirou; (Tokyo, JP) |
Assignee: |
YAMATAKE CORPORATION
Tokyo
JP
|
Family ID: |
44370588 |
Appl. No.: |
13/089418 |
Filed: |
April 19, 2011 |
Current U.S.
Class: |
702/45 |
Current CPC
Class: |
G01F 1/60 20130101 |
Class at
Publication: |
702/45 |
International
Class: |
G01F 1/58 20060101
G01F001/58 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2010 |
JP |
2010-098647 |
Claims
1. An electromagnetic flow meter comprising: a magnetic excitation
coil arranged so that the direction in which the magnetic field
thereof is produced is perpendicular to the direction of flow of a
fluid flowing within a measuring tube; a magnetic excitation device
providing a magnetic excitation electric current to the magnetic
excitation coil with the polarity thereof switching alternatingly;
a pair of electrodes disposed within the measuring tube
perpendicular to the direction of flow of the fluid flowing within
the measuring tube and to the direction of the magnetic field
produced by the magnetic excitation coil; a differential amplifier
performing differential amplification of a signal EMF produced
between the electrodes, to produce an AC flow rate signal; an AC
amplifier amplifying the AC flow rate signal from the differential
amplifier; a sampler sampling the AC flow rate signal that has been
amplified by the AC amplifier, to produce a DC flow rate signal; a
DC amplifier amplifying the DC flow rate signal from the sampler;
an A/D converter converting into a digital signal the DC flow rate
signal amplified by the DC amplifier; and a processor calculating a
flow rate of the fluid flowing within the measuring tube from the
digital signal converted by the A/D converter; wherein the DC
amplifier comprises: first through Nth (where N.gtoreq.2)
individual gain generators inputting individually the DC flow rate
signal from the sampler and for applying, to the DC flow rate
signal, the gain produced thereby, established so that the gains of
the first through Nth individual gain generators applied to the DC
flow rate signal are as sequentially larger values; a gain selector
selecting, as the DC flow rate signal to the A/D converter, one of
the outputs from the first through Nth individual gain generators;
and a saturation preventer preventing the occurrence of saturation
in a later stage individual gain generator, connected to all of the
earlier-stage individual gain generators of the first through Nth
individual gain generators except for the first individual gain
generator; and wherein the processor comprises: a controller
controlling enabling/disabling of the saturation preventing
operation of the saturation preventer in the DC amplifier and for
controlling a selecting operation of the gain selector, based on
the calculated flow rate of the fluid.
2. The electromagnetic flow meter as set forth in claim 1, wherein
the controller, when controlling the selecting operation by the
gain selector, sets the individual gain generators with the output
thereof selected by the gain selector as the used gain generator,
sets to disabled the saturation preventing operations of the
saturation preventer that are connected to the individual gain
generators that generate gains equal to or less than the gain
generated by the used gain generator, and set to enabled the
saturation preventing operations of the other saturation
preventer.
3. The electromagnetic flow meter as set forth in claim 2, wherein:
the saturation preventer set a state wherein the saturation
preventing operation in a state wherein the DC flow rate signal
from the sampler is applied to a later-stage individual gain
generator is disabled, and set a state wherein the saturation
preventing operation in a state wherein a reference voltage, which
is set in advance, is applied to the individual gain generator is
enabled; and the value of the reference voltage in the saturation
preventer is established as a value corresponding to the value of
the DC flow rate signal to the used gain generator immediately
prior to the saturation preventing operation thereof switching from
enabled to disabled.
4. The electromagnetic flow meter as set forth in claim 1, wherein
the saturation preventers are connected also to a stage prior to
the first individual gain generators.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn.119 to Japanese Patent Application No. 2010-098647, filed
Apr. 22, 2010, which is incorporated herein by reference.
FIELD OF TECHNOLOGY
[0002] The present invention relates to an electromagnetic flow
meter for measuring a flow rate of a fluid having electrical
conductivity in various types of process systems.
BACKGROUND OF THE INVENTION
[0003] Conventionally, in this type of electromagnetic flow meter,
an excitation current with a polarity that switches alternatingly
is supplied to an excitation coil that is disposed so that the
direction wherein the magnetic field thereof is produced is
perpendicular to the direction of flow of a fluid flowing within a
measuring tube, and a signal EMF that is produced between a pair of
electrodes that are disposed within the measuring tube,
perpendicular to the magnetic field that is produced by the
excitation coil, is detected, and this signal EMF that is produced
between the electrodes is differentially amplified to be an AC flow
rate signal, where this AC flow rate signal is further amplified,
sampled, and subjected to the signal processing to produce a
measured flow rate.
[0004] This type of electromagnetic flow meter can be broadly
categorized into battery-type electromagnetic flow meters,
two-wire-type electromagnetic flow meters, and four-wire-type
electromagnetic flow meters, based on differences in the driving
power supply systems. Additionally, these electromagnetic flow
meters have the property of having high measurement accuracy
because the signal EMF that is produced between the electrodes is
larger in accordance with the flow of the excitation current.
[0005] Because the four-wire-type electromagnetic flow meter has
electric power supplied to the electromagnetic flow meter through
two power supply wires that are separate from the two signal wires,
it is possible to increase the excitation current flowing in the
excitation coil regardless of the flow rate being measured. In
contrast, in the two-wire-type electromagnetic flow meter, electric
power that has a self-linking effect is generated from the 4 to 20
mA electric current signal that is sent through the two signal
wires, and thus it is not possible to increase the excitation
current that can flow in the excitation coil. Moreover, in the
battery-type electromagnetic flow meter, the power for driving
depends on the built-in battery power supply, and thus the
excitation current, of necessity, must be small.
[0006] In this way, the excitation current in a two-wire-type
electromagnetic flow meter or in a battery-type electromagnetic
flow meter is a small electric current when compared to that of the
four-wire-type electromagnetic flow meter, and thus the signal EMF
that is obtained between the electrodes is smaller. Because of
this, the gain in the signal amplifying circuit in the
two-wire-type electromagnetic flow meter and the battery-type
electromagnetic flow meter is set so as to be large compared to
that of the four-wire-type electromagnetic flow meter.
Additionally, the signal EMF that is produced between the
electrodes has a magnitude that increases with an increase in the
flow rate (the speed of flow) of the fluid being measured, and thus
switching the gain of the signal amplifying circuit in accordance
with the flow rate, or in other words, having the gain the low when
the flow rate is high and having the gain the high when the flow
rate is low, makes it possible to increase the accuracy depending
on the range of the flow rate being measured.
[0007] FIG. 10 illustrates schematically an electromagnetic flow
meter that is provided with a function for switching automatically
the gain of the signal amplifying circuit in accordance with the
flow rate (See, for example, Japanese Unexamined Patent Application
Publication H6-258111). In this figure: 100 is a detecting device
for receiving a magnetic excitation electric current Iex, applying
a magnetic field to a fluid that flows within a measuring tube 1C,
and detecting the signal EMF that is produced; and 200 is a
converting device for not only applying the magnetic excitation
electric current Iex to the detecting device 100, but also
processing the signal EMF from the detecting device 100 to measure
the flow rate of the fluid flowing within the measuring tube
1C.
[0008] In this electromagnetic flow meter, the converting device
200 has a differential amplifying circuit 2; an AC amplifying
circuit 3; a sample hold circuit 4; a DC amplifying circuit 5; an
A/D converting circuit 6; a processing portion 7; and a magnetic
excitation circuit 8. Additionally, the detecting device 100 is
provided with: a magnetic excitation coil 1D arranged so that the
direction in which the magnetic field thereof is produced is
perpendicular to the direction of flow of the fluid flowing within
the measuring tube 1C; and a pair of electrodes 1A and 1B disposed
perpendicular to the direction of flow of the fluid flowing within
the measuring tube 1C and to the direction of the magnetic field
produced by the magnetic excitation coil 1D.
[0009] In this electromagnetic flow meter, the magnetic excitation
circuit 8 outputs a square wave AC magnetic excitation electric
current Iex of a specific frequency based on an instruction from
the processing portion 7. The magnetic excitation coil 1D is
excited magnetically by the magnetic excitation electric current
Iex from the magnetic excitation circuit 8, to produce a magnetic
field, where the magnetic field that is produced is applied to the
fluid flowing within the measuring tube 1C. This produces a signal
EMF between the electrodes 1A and 1B with an amplitude that is in
accordance with the speed of flow of the fluid. The signal EMF that
is produced between the electrodes 1A and 1B is inputted into the
differential amplifying circuit 2.
[0010] The differential amplifying circuit 2 performs differential
amplification on the signal EMF produced between the electrodes 1 A
and 1B, to produce an AC flow rate signal. This AC flow rate signal
is amplified by the AC amplifying circuit 3, and applied to the
sample hold circuit 4. The sample hold circuit 4 samples the AC
flow rate signal that is amplified by the AC amplifying circuit 3,
to produce a DC flow rate signal. This DC flow rate signal is
amplified by the DC amplifying circuit 5, and applied to A/D
converting circuit 6. The A/D converting circuit 6 converts into a
digital signal the DC flow rate signal that has been amplified by
the DC amplifying circuit 5, and sends it to the processing portion
7. The processing portion 7 calculates the flow rate of the fluid
flowing within the measuring tube 1C from the digital signal from
the A/D converting circuit 6, and outputs the calculated flow rate
as the measured flow rate. Additionally, the processing portion 7
switches the gain in the AC amplifying circuit 3 in accordance with
the calculated measured flow rate, that is, switches the gain that
is applied to the AC flow rate signal from the differential
amplifying circuit 2. In this case, there is a low gain when the
measured flow rate is high, and a high gain when the measured flow
rate is low.
[0011] However, in the conventional electromagnetic flow meter
illustrated in FIG. 10, the gain of the AC amplifying circuit 3,
which is a stage prior to the sample hold circuit 4, is switched,
and thus when low frequency noise, or the like, is produced when
there is a high gain when the flow rate is low (that is, when low
frequency noise is produced in, for example, a case wherein a solid
object within the flow strikes the measuring electrode), there is
an increased likelihood that the operational amplifier in the AC
amplifying circuit 3 will become saturated, and thus there is the
risk that there will be error in the measured flow rate due to the
occurrence of saturation.
[0012] The present invention was created in order to solve the
problem as set forth above, and the object thereof is to provide an
electromagnetic flow meter able to increase the accuracy of the
measured flow rate, without producing saturation.
SUMMARY OF THE INVENTION
[0013] In order to achieve such an object, the present invention is
an electromagnetic flow meter comprising: a magnetic excitation
coil arranged so that the direction in which the magnetic field
thereof is produced is perpendicular to the direction of flow of a
fluid flowing within a measuring tube; magnetic excitation means
for providing a magnetic excitation electric current to the
magnetic excitation coil with the polarity thereof switching
alternatingly; a pair of electrodes disposed within the measuring
tube perpendicular to the direction of flow of the fluid flowing
within the measuring tube and to the direction of the magnetic
field produced by the magnetic excitation coil; differential
amplifier for performing differential amplification of a signal EMF
produced between the electrodes, to produce an AC flow rate signal;
AC amplifier for amplifying the AC flow rate signal from the
differential amplifier; sampler for sampling the AC flow rate
signal that has been amplified by the AC amplifier, to produce a DC
flow rate signal; DC amplifier for amplifying the DC flow rate
signal from the sampler; A/D converter for converting into a
digital signal the DC flow rate signal amplified by the DC
amplifier; and processor for calculating a flow rate of the fluid
flowing within the measuring tube from the digital signal converted
by the AD converter; wherein the DC amplifier are provided with:
first through Nth (where N.gtoreq.2) individual gain generator for
inputting individually the DC flow rate signal from the sampler and
for applying, to the DC flow rate signal, the gain produced
thereby, established so that the gains of the first through Nth
individual gain generators applied to the DC flow rate signal are
as sequentially larger values; gain selector for selecting, as the
DC flow rate signal to the A/D converter, one of the outputs from
the first through Nth individual gain generator; and saturation
preventer for preventing the occurrence of saturation in a
later-stage individual gain generator, connected to all of the
earlier-stage individual gain generator of the first through Nth
individual gain generator except for the first individual gain
generator; and wherein: the processor are provided with controller
for controlling enabling/disabling of the saturation preventing
operation of the saturation preventer in the DC amplifier, and for
controlling a selecting operation of the gain selector, based on
the calculated flow rate of the fluid.
[0014] In the present invention, if, for example, N=3, the DC
amplifier are provided with first individual gain generator for
generating a gain G1, second individual gain generator for
generating a gain G2 (wherein G2>G1), and third individual gain
generator for generating a gain G3 (where G3>G2). Additionally,
the saturation preventer for preventing the occurrence of
saturation in the second individual gain generator is provided in
an earlier stage than the second individual gain generator, and the
saturation preventer for preventing the occurrence of saturation in
the third individual gain generator is provided in an earlier stage
than the third individual gain generator. If here the saturation
preventer provided in the earlier stage than the second individual
gain generator are defined as the second saturation preventer, and
the saturation preventer provided in the earlier stage than the
third individual gain generator are defined as the third saturation
preventer, the enabling/disabling of the saturation preventing
operations of the second saturation preventer and the third
saturation preventer, and the selecting operation of the gain
selector, are controlled by the controller.
[0015] For example, when controlling the selecting operation of the
gain selector in the present invention, the individual gain
generator for which the output is selected are defined as the used
gain selector. Additionally, the saturation preventing operations
of the saturation preventer connected to the individual gain
generating circuits that generate gains that are less than or equal
to the gain that is generated by the used gain generator are
disabled and the saturation preventing operations of the other
saturation preventer are enabled. In terms of the aforementioned
example wherein N=3, if the second individual gain generator are
defined as the used gain generator, then the gain preventing
operation of the second saturation preventer, which are connected
to the second individual gain generator, are disabled, and the gain
preventing operation of the third saturation preventer, which are
connected to the third individual gain generator, are enabled. In
this case, the output from the second individual gain generator
(the used gain generator) is selected as the DC flow rate signal to
the A/D converter, but because the saturation preventing operation
of the third saturation preventer is enabled, there will be no
saturation, by the third individual gain generator, even when the
value of the DC flow rate signal from the sampler is large.
[0016] In the present invention, the gain switching is performed by
the DC amplifier, rather than by the AC amplifier. Additionally,
the appropriate control of the enabling/disabling of the saturation
preventing operations of the saturation preventer, and the
selecting operation for the gain selector, by the DC amplifier
makes it possible to prevent the occurrence of saturation in the
individual gain generator that generate higher gains than the gain
generated by the used gain generator. Doing so causes constant
amplification by the AC amplifier and prevents the occurrence of
saturation in both the AC amplifier and the DC amplifier, thereby
enabling an increase in the accuracy of the measured flow rate.
[0017] Note that as an alternative example of the present
invention, one may consider connecting saturation preventer to an
earlier stage than the first individual gain generator.
Additionally, in the present invention the enabling/disabling of
the saturation preventing operations by the saturation preventer
are controlled by the controller, and, for example control is
performed with the state wherein the DC flow rate signal from the
sampler is applied to the later-stage individual gain generator
defined as the state wherein the saturation preventing operation is
disabled, and with the state wherein a reference voltage,
established in advance, is applied to the later-stage individual
gain generator defined as the state wherein the saturation
preventing operation is enabled, in this case, the value of the
reference voltage in the saturation preventer is established as a
value corresponding to the value of the DC flow rate signal to the
used gain generator immediately prior to the saturation preventing
operation thereof switching from enabled to disabled.
[0018] In the present invention, the gain switching is performed by
the DC amplifier instead of the AC amplifier, and the DC amplifier
are provided with first through Nth individual gain generator, gain
selector, and saturation preventer, and enabling/disabling the
saturation preventing operations of the saturation preventer in the
DC amplifier, and the selecting operation for the gain selector,
are performed based on the calculated flow rate, and thus the AC
amplifier perform constant amplification and there is no saturation
in either the AC amplifier or the DC amplifier, thus making it
possible to improve the accuracy of the measured flow rate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram illustrating schematically an example of
an electromagnetic flow meter according to the present
invention.
[0020] FIG. 2 is a diagram illustrating schematically the internal
structure (in a state wherein the gain G3 is selected) of an
example of a DC amplifying circuit in this electromagnetic flow
meter.
[0021] FIG. 3 is a diagram illustrating the state wherein the gain
G2 is selected in the example.
[0022] FIG. 4 is a diagram illustrating the state wherein the gain
G1 is selected in the example.
[0023] FIG. 5 is a diagram illustrating the situation wherein the
switching of the input voltage to the A/D converting circuit and
the switching of the gain by the DC amplifying circuit is performed
when the flow rate goes up (when the flow rate increases).
[0024] FIG. 6 is a diagram illustrating the situation wherein the
switching of the input voltage to the A/D converting circuit and
the switching of the gain by the DC amplifying circuit is performed
when the flow rate goes down (when the flow rate decreases).
[0025] FIG. 7 is a diagram illustrating schematically the internal
structure (in a state wherein the gain G3 is selected) of another
example of a DC amplifying circuit in an electromagnetic flow meter
according to present invention.
[0026] FIG. 8 is a diagram illustrating the state wherein the gain
G2 is selected in the other example.
[0027] FIG. 9 is a diagram illustrating the state wherein the gain
G1 is selected in the other example.
[0028] FIG. 10 is a diagram illustrating schematically a
conventional electromagnetic flow meter.
DETAILED DESCRIPTION OF THE INVENTION
[0029] An example is explained below in detail, based on the
drawings. FIG. 1 is a diagram illustrating schematically an
electromagnetic flow meter according to the present invention. In
this figure, codes that are the same as those in FIG. 10 indicate
identical or equivalent structural elements as the structural
elements explained in reference to FIG. 10, and explanations
thereof are omitted.
[0030] In the conventional electromagnetic flow meter illustrated
in FIG. 10, the gain switching in the AC amplifying circuit 3 was
performed by a command from the processing portion 7. However, in
the electromagnetic flow meter according to the example, constant
amplification is performed in the AC amplifying circuit 3, and the
gain switching is performed by the DC amplifying circuit 5 through
a command from the processing portion 7.
[0031] Note that in FIG. 1 the DC amplifying circuit 5 is defined
as 5A, to distinguish from the conventional DC amplifying circuit 5
illustrated in FIG. 10. Additionally the processing portion 7 is
defined as 7A, to distinguish from the conventional processing
portion 7 illustrated in FIG. 10.
Example Wherein No Reference Voltage is Set in the Saturation
Preventing Circuit
[0032] FIG. 2 illustrates schematically the internal structure of
an example of the DC amplifying circuit 5A. In this example, the DC
amplifying circuit 5A includes individual gain generating circuits
51-1 through 51-3 that each input the DC flow rate signal from the
sample hold circuit 4, and each applies the gain generated thereby
to the DC flow rate signal to produce an output; a gain selecting
circuit 52 for selecting, as the DC flow rate signal to the A/D
converting circuit 6, one of the outputs from the individual gain
generating circuits 51-1 through 51-3; and saturation preventing
circuits 53-2 and 53-3, connected respectively to the earlier
stages of the individual gain generating circuits 51-2 and
51-3.
[0033] In the DC amplifying circuit 5A, the individual gain
generating circuits 51 (51-1 through 51-3) have operational
amplifiers OP (OP1 through OP3), low-pass filters LPF (LPF1 through
LPF3), resistances R1 (R11 through R13), and resistances R2 (R21
through R23). If the gains generated in these individual gain
generating circuits 51-1, 51-2, and 51-3, are, respectively, G1,
G2, and the G3, the magnitudes of the gains are set to
G1<G2<G3. That is, the gains G1, G2, and G3 of the individual
gain generating circuits 51-1, 51-2, and 51-3 are established as
sequentially larger values (small gain, medium gain, and large
gain).
[0034] Additionally, the gain selecting circuit 52 is structured
from a first switch SW1 and a second switch SW2, where the switches
SW1 and SW2 are both switches that are single circuits with two
contact points. In the gain selecting circuit 52, the contact point
S1a of the switch SW1 is connected to an output line L1 from the
individual gain generating circuit 51-1, and the contact point S1b
of the switch SW1 is connected to an output line L2 from the
individual gain generating circuit 51-2. Additionally, the common
terminal S1c of the switch SW1 is connected to the contact point
S2a of the switch SW2, and the contact point S2b of the switch SW2
is connected to an output line L3 from the individual gain
generating circuit 51-3, and the contact point S2c of the switch
SW2 is connected to an output line LOUT from the A/D converting
circuit 6.
[0035] Moreover, the saturation preventing circuits 53 (53-2 and
53-3) are also single circuits having two contact points, as with
the switches SW1 and SW2. In the saturation preventing circuit
53-2, the contact point T2a is connected to an input line LIN from
the sample hold circuit 4, the contact point T2b is open, and the
common terminal T2c is connected to the + side input terminal of
the operational amplifier OP2 of the individual gain generating
circuit 51-2. In the saturation preventing circuit 53-3, the
contact point T3a is connected to an input line LIN from the sample
hold circuit 4, the contact point T3b is open, and the common
terminal T3c is connected to the + side input terminal of the
operational amplifier OP3 of the individual gain generating circuit
51-3.
[0036] In the individual gain generating circuit 51-1 the + side
input of the operational amplifier OP1 is connected directly to the
input line LIN from the sample hold circuit 4. That is, the
saturation preventing circuit 53 is not provided at the
earlier-stage of the operational amplifier OP1. A series-connected
circuit of the resistance R11 and the resistance R21 is connected
to the output line from the operational amplifier OP1 to the
low-pass filter LPF1, and the - side input of the operational
amplifier OP1 is connected to the contact point between the
resistance R11 and the resistance R21.
[0037] In the individual gain generating circuit 51-2 as well,
similarly, a series-connected circuit of the resistance R12 and the
resistance R22 is connected to the output line from the operational
amplifier OP2 to the low-pass filter LPF2, and the - side input of
the operational amplifier OP2 is connected to the contact point
between the resistance R12 and the resistance R22. In the
individual gain generating circuit 51-3 as well, similarly, a
series-connected circuit of the resistance R13 and the resistance
R23 is connected to the output line from the operational amplifier
OP3 to the low-pass filter LPF3, and the - side input of the
operational amplifier OP3 is connected to the contact point between
the resistance R13 and the resistance R23.
[0038] In this DC amplifying circuit 5A the operations of the
switches SW1 and SW2 in the gain selecting circuit 52 are
controlled by gain selecting instructions sent from the processing
portion 7. Moreover, the operations of the saturation preventing
circuits 53-2 and 53-3 are controlled by saturation preventing
operation enable/disable setting commands sent from the processing
portion 7. In the present example, the instructions combining the
gain selecting instructions and the saturation preventing operation
enabling/disabling, setting instructions are known as the gain
switching instructions from the processing portion 7A.
[0039] The processing portion 7A is achieved through hardware,
including a processor and a storage device, and a program that
works in conjunction with this hardware to produce various
functions, and has a gain switching instructions generating
function, to the DC amplifying circuit 5A, as a function that is
unique to the present example. The gain switching instructions
generating function of the processing portion 7A and the operation
in the DC amplifying circuit 5A in accordance with that gain
switching instruction will be explained below taking, as examples,
a case wherein the flow rate gets larger (the flow speed increases)
and a case wherein the flow rate gets smaller (the flow speed
decreases).
The Example Wherein the Flow Rate Gets Larger (Wherein the Flow
Speed Increases)
Small Flow Rate (Low Flow Speed)
[0040] At this point, the flow rate is small, where the flow speed
is, for example, no more than 0.32 m/s (a point between t0 and t1,
shown in FIG. 5). In this case, the processing portion 7A generates
a gain switching instruction to the gain G3, which is the large
gain of the gains G1, G2, and G3, and sends it to the DC amplifying
circuit 5A.
[0041] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1b side and
of the switch SW2 to the contact point S2b side, sends a saturation
preventing operation disabling setting instruction to the
saturation preventing circuit 53-2 for the connecting mode for the
contact point T2a side, and sends a saturation preventing operation
disabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3a side.
[0042] As a result, as illustrated in FIG. 2, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2b side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3a side.
[0043] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-3 from the sample hold
circuit 4 through the saturation preventing circuit 53-3, the gain
G3 produced by the individual gain generating circuit 51-3 is
applied to the DC flow rate signal, and the DC flow rate signal to
which the gain G3 has been applied is outputted to the A/D
converting circuit 6 of the next stage through the switch SW2 of
the gain selecting circuit 52.
[0044] While in this case the DC flow rate signal is inputted also
into the individual gain generating circuits 51-1 and 51-2 from the
sample hold circuit 4, in the gain selecting circuit 52 the switch
SW1 is set to the connecting mode for the contact point S1b side,
and the switch SW2 is set to be connecting mode for the contact
point S2b side, so the DC flow rate signals that are amplified by
the individual gain generating circuits 51-1 and 51-2 do not pass
through the gain selecting circuit 52, so are not outputted to the
A/D converting circuit 6 of the next stage.
[0045] Additionally, in this case the individual gain generating
circuit 51-3 is selected as the used gain generating circuit, so
the output from the individual gain generating circuit 51-3 is
outputted to the A/D converting circuit 6 in the next stage, but
because the DC flow rate signals from the sample hold circuit 4
that are inputted into the individual gain generating circuits 51-1
and 51-2 are small, there is no saturation in the individual gain
generating circuits 51-1 and 51-2.
Medium Flow Rate
[0046] When the flow rate increases and the flow speed exceeds 0.32
m/s (the point t1 shown in FIG. 5), the processing portion 7A
generates a gain switching instruction to the gain G2, which is the
medium gain of the gains G1, G2, and G3, and sends it to the DC
amplifying circuit 5A. In this case, the processing portion 7A
sends a gain selecting instruction to the gain selecting circuit 52
for the connecting mode of the switch SW1 to the contact point S1b
side and of the switch SW2 to the contact point S2a side, sends a
saturation preventing operation disabling setting instruction to
the saturation preventing circuit 53-2 for the connecting mode for
the contact point T2a side, and sends a saturation preventing
operation enabling setting instruction to the saturation preventing
circuit 53-3 for the connecting mode for the contact point T3b
side.
[0047] As a result, as illustrated in FIG. 3, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0048] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-2 from the sample hold
circuit 4 through the saturation preventing circuit 53-2, the gain
G2 produced by the individual gain generating circuit 51-2 is
applied to the DC flow rate signal, and the DC flow rate signal to
which the gain G2 has been applied is outputted to the A/D
converting circuit 6 of the next stage through the switches SW1 and
5W2 of the gain selecting circuit 52.
[0049] In this case the individual gain generating circuit 51-2 is
selected as the used gain generating circuit, so the output from
the individual gain generating circuit 51-2 is outputted to the A/D
converting circuit 6 in the next stage, but because the saturation
preventing circuit 53-3 is set to the connecting mode for the
contact point T3b, input into the individual gain generating
circuit 51-3 is blocked the saturation preventing operation is
enabled), so there is no saturation of the individual gain
generating circuit 51-3 even when the value of the DC flow rate
signal from the sample hold circuit 4 becomes large.
[0050] That is, if the saturation preventing circuit 53-3 were
still set to the connecting mode for the contact point side T3a
(with the saturation preventing operation disabled), then because
the value of the DC flow rate signal from the sample hold circuit 4
is large, the DC flow rate signal would be amplified by the large
gain G3, which would cause saturation in the individual gain
generating circuit 51-3. In this case, the saturation of the
individual gain generating circuit 51-3 would have an effect on the
input into the individual gain generating circuit 51-2, which is
the used gain generating circuit, so that effect would propagate to
the DC flow rate signal to the A/D converting circuit 6. So that
this does not happen, in the present example, the saturation
preventing operation of the saturation preventing circuit 53-3 that
is provided for the individual gain generating circuit 51-3 is
enabled when the individual gain generating circuit 51-2 is
selected as the used gain generating circuit.
Large Flow Rate
[0051] When the flow rate increases further and the flow speed
exceeds 2.2 m/s (the point t2 shown in FIG. 5), the processing
portion 7A generates a gain switching instruction to the gain G1,
which is the small gain of the gains G1, G2, and G3, and sends it
to the DC amplifying circuit 5A.
[0052] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1a side and
of the switch SW2 to the contact point S2a side, sends a saturation
preventing operation enabling setting instruction to the saturation
preventing circuit 53-2 for the connecting mode for the contact
point T2b side, and sends a saturation preventing operation
enabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3b side.
[0053] As a result, as illustrated in FIG. 4, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1a side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2b side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0054] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-1 from the sample hold
circuit 4, the gain G1 produced by the individual gain generating
circuit 51-1 is applied to the DC flow rate signal, and the DC flow
rate signal to which the gain G1 has been applied is outputted to
the A/D converting circuit 6 of the next stage through the switches
SW1 and SW2 of the gain selecting circuit 52.
[0055] In this case the individual gain generating circuit 51-1 is
selected as the used gain generating circuit, so the output from
the individual gain generating circuit 51-1 is outputted to the A/D
converting circuit 6 in the next stage, but because the saturation
preventing circuit 53-2 is set to the connecting mode for the
contact point T2b side, which is open, and the saturation
preventing circuit 53-3 is set to the connecting mode for the
contact point T3b (saturation preventing operation is enabled),
there is no saturation of the individual gain generating circuits
51-2 and 51-3 even when the value of the DC flow rate signal from
the sample hold circuit 4 becomes large, so there is no impact on
the DC flow rate signal to the All) converting circuit 6.
The Example Wherein the Flow Rate Gets Smaller (Wherein the Flow
Speed Decreases)
Large Flow Rate (High Flow Speed)
[0056] At this point, the flow rate is large, where the flow speed
is, for example, no less than 1.6 m/s (a point between t2 and t3,
shown in FIG. 6). In this case, the processing portion 7A generates
a gain switching instruction to the gain G1, which is the small
gain of the gains G1, G2, and G3, and sends it to the DC amplifying
circuit 5A.
[0057] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1a side and
of the switch SW2 to the contact point S2a side, sends a saturation
preventing operation enabling setting instruction to the saturation
preventing circuit 53-2 for the connecting mode for the contact
point T2b side, and sends a saturation preventing operation
enabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3b side.
[0058] As a result, as illustrated in FIG. 4, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1a side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2b side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0059] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-1 from the sample hold
circuit 4, the gain G1 produced by the individual gain generating
circuit 51-1 is applied to the DC flow rate signal, and the DC flow
rate signal to which the gain G1 has been applied is outputted to
the A/D converting circuit 6 of the next stage through the switches
SW1 and SW2 of the gain selecting circuit 52.
[0060] In this case the individual gain generating circuit 51-1 is
selected as the used gain generating circuit, so the output from
the individual gain generating circuit 51-1 is outputted to the A/D
converting circuit 6 in the next stage, but because the saturation
preventing circuit 53-2 is set to the connecting mode for the
contact point T2b side, and the saturation preventing circuit 53-3
is set to the connecting mode for the contact point T3b (saturation
preventing operation is enabled), there is no saturation of the
individual gain generating circuits 51-2 and 51-3 even when the
value of the DC flow rate signal from the sample hold circuit 4
becomes large, so there is no impact on the DC flow rate signal to
the A/D converting circuit 6.
Medium Flow Rate
[0061] When the flow rate decreases and the flow speed falls below
1.6 m/s (the point t2 shown in FIG. 6), the processing portion 7A
generates a gain switching instruction to the gain G2, which is the
medium gain of the gains G1, G2, and G3, and sends it to the DC
amplifying circuit 5A.
[0062] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1b side and
of the switch SW2 to the contact point S2a side, sends a saturation
preventing operation disabling setting instruction to the
saturation preventing circuit 53-2 for the connecting mode for the
contact point T2a side, and sends a saturation preventing operation
enabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3b side.
[0063] As a result, as illustrated in FIG. 3, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0064] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-2 from the sample hold
circuit 4 through the saturation preventing circuit 53-2, the gain
G2 produced by the individual gain generating circuit 51-2 is
applied to the DC flow rate signal, and the DC flow rate signal to
which the gain G2 has been applied is outputted to the A/D
converting circuit 6 of the next stage through the switches SW1 and
SW2 of the gain selecting circuit 52.
[0065] In this case the individual gain generating circuit 51-2 is
selected as the used gain generating circuit, so the output from
the individual gain generating circuit 51-2 is outputted to the A/D
converting circuit 6 in the next stage, but because the saturation
preventing circuit 53-3 is set to the connecting mode for the
contact point T3b (saturation preventing operation is enabled),
there is no saturation of the individual gain generating circuits
51-2 and 51-3 even when the value of the DC flow rate signal from
the sample hold circuit 4 becomes large, so there is no impact on
the DC flow rate signal to the A/D converting circuit 6.
Small Flow Rate
[0066] When the flow rate further decreases and the flow speed
falls below 0.22 m/s (the point t1 shown in FIG. 6), the processing
portion 7A generates a gain switching instruction to the gain G3,
which is the large gain of the gains G1, G2, and G3, and sends it
to the DC amplifying circuit 5A.
[0067] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1b side and
of the switch SW2 to the contact point S2b side, sends a saturation
preventing operation disabling setting instruction to the
saturation preventing circuit 53-2 for the connecting mode for the
contact point T2a side, and sends a saturation preventing operation
disabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3a side.
[0068] As a result, as illustrated in FIG. 2, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2b side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3a side.
[0069] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-3 from the sample hold
circuit 4, the gain G3 produced by the individual gain generating
circuit 51-3 is applied to the DC flow rate signal, and the DC flow
rate signal to which the gain G3 has been applied is outputted to
the A/D converting circuit 6 of the next stage through the switch
SW2 of the gain selecting circuit 52.
[0070] While in this case the DC flow rate signal is inputted also
into the individual gain generating circuits 51-1 and 51-2 from the
sample hold circuit 4, in the gain selecting circuit 52 the switch
SW1 is set to the connecting mode for the contact point S1b side,
and the switch SW2 is set to be connecting mode for the contact
point S2b side, so the DC flow rate signals that are amplified by
the individual gain generating circuits 51-1 and 51-2 do not pass
through the gain selecting circuit 52, so are not outputted to the
A/D converting circuit 6 of the next stage.
[0071] Additionally, in this case the individual gain generating
circuit 51-3 is selected as the used gain generating circuit, so
the output from the individual gain generating circuit 51-3 is
outputted to the A/D converting circuit 6 in the next stage, but
because the DC flow rate signals from the sample hold circuit 4
that are inputted into the individual gain generating circuits 51-1
and 51-2 are small, there is no saturation in the individual gain
generating circuits 51-1 and 51-2, and there is no effect on the DC
flow rate signal to the A/D converting circuit 6.
Example Wherein a Reference Voltage is Set in the Saturation
Preventing Circuit
[0072] In the previous example, in the saturation preventing
circuits 53-2 and 53-3 the saturation preventing operations were
enabled through setting the connecting modes to the contact point
T2b and T3b sides, which were open. In this case, when switching
the gain from G1 to G2 (the point t2 shown in FIG. 6) in a process
wherein the flow rate becomes smaller (the flow speed becomes
lower), the DC flow rate signal is inputted suddenly from the
sample hold circuit 4 from a state wherein the individual gain
generating circuit 51-2 was open, and thus there is a delay in the
amplification process in the individual gain generating circuit
51-2, which produces switching error. Similarly, when switching the
gain from G2 to G3 (the point t1 shown in FIG. 6), the DC flow rate
signal is inputted suddenly from the sample hold circuit 4 from a
state wherein the individual gain generating circuit 51-3 was open,
and thus there is a delay in the amplification process in the
individual gain generating circuit 51-3, which produces switching
error.
[0073] Note that the aforementioned switching error is not produced
in a process wherein the flow rate becomes larger (the flow speed
becomes greater). That is, when switching the gain from G3 to G2
(the point t1 shown in FIG. 5, FIG. 2.fwdarw.FIG. 3) in a process
wherein the flow rate becomes larger, the state is one wherein the
saturation preventing operation of the saturation preventing
circuit 53-2 disabled and the DC flow rate signal from the sample
hold circuit 4 is already inputted into the individual gain
generating circuit 51-2, and thus there is no delay in the
amplification process in the individual gain generating circuit
51-2, so no switching error is produced. Similarly, when switching
the gain from G2 to G1 (the point t2 shown in FIG. 5, FIG.
3.fwdarw.FIG. 4), the state is one wherein the DC flow rate signal
is already inputted from the sample hold circuit 4 into the
individual gain generating circuit 51-1, and thus there is no delay
in the amplification process in the individual gain generating
circuit 51-1, and no switching error will be produced,
[0074] In this way, while there is no problem in the example above
in a process wherein the flow rate is increasing, there will be
switching error in a process wherein the flow rate is decreasing.
Given this, in the present example a reference voltage E2 is
connected to the contact point T2b of the saturation preventing
circuit 53-2, and a reference voltage E3 is connected to the
contact point T3b of the saturation preventing circuit 53-3, as
illustrated in FIG. 7, in order to prevent the occurrence of
switching error in a process wherein the flow rate is
decreasing.
[0075] In this case, the reference voltage E2 in the saturation
preventing circuit 53-2 is established as a value corresponding to
the value of the DC flow rate signal to the individual gain
generating circuit 51-1 in consideration of the value of the DC
flow rate signal to the individual gain generating circuit 51-1,
which is the gain generating circuit used immediately prior to the
switching of the saturation preventing operation of the saturation
preventing circuit 53-2 from enabled to disabled (FIG.
4.fwdarw.FIG. 3). That is, the reference voltage E2 is established
as a value that is equal to the voltage value of the DC flow rate
signal that is expected through anticipating the voltage value of
the DC flow rate signal from the sample hold circuit 4 immediately
prior to the used gain generating circuit being switched from the
individual gain generating circuit 51-1 to the individual gain
generating circuit 51-2.
[0076] Additionally, the reference voltage E3 in the saturation
preventing circuit 53-3 is established as a value corresponding to
the value of the DC flow rate signal to the individual gain
generating circuit 51-2 in consideration of the value of the DC
flow rate signal to the individual gain generating circuit 51-2,
which is the gain generating circuit used immediately prior to the
switching of the saturation preventing operation of the saturation
preventing circuit 53-3 from enabled to disabled (FIG.
3.fwdarw.FIG. 2). That is, the reference voltage E3 is established
as a value that is equal to the voltage value of the DC flow rate
signal that is expected through anticipating the voltage value of
the DC flow rate signal from the sample hold circuit 4 immediately
prior to the used gain generating circuit being switched from the
individual gain generating circuit 51-2 to the individual gain
generating circuit 51-3.
The Example Wherein the Flow Rate Gets Smaller (Wherein the Flow
Speed Decreases)
Large Flow Rate (High Flow Speed)
[0077] At this point, the flow rate is large, where the flow speed
is, for example, no less than 1.6 m/s (a point between t2 and t3,
shown in FIG. 6). In this case, the processing portion 7A generates
a gain switching instruction to the gain G1, which is the small
gain of the gains G1 G2, and G3, and sends it to the DC amplifying
circuit 5A.
[0078] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1a side and
of the switch SW2 to the contact point S2a side, sends a saturation
preventing operation enabling setting instruction to the saturation
preventing circuit 53-2 for the connecting mode for the contact
point T2b side, and sends a saturation preventing operation
enabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3b side.
[0079] As a result, as illustrated in FIG. 9, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1a side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2b side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0080] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-1 from the sample hold
circuit 4, the gain G1 produced by the individual gain generating
circuit 51-1 is applied to the DC flow rate signal, and the DC flow
rate signal to which the gain G1 has been applied is outputted to
the A/D converting circuit 6 of the next stage through the switches
SW1 and SW2 of the gain selecting circuit 52. In this case the
individual gain generating circuit 51-1 is selected as the used
gain generating circuit, so the output from the individual gain
generating circuit 51-1 is outputted to the A/D converting circuit
6 in the next stage, but because the saturation preventing circuit
53-2 is set to the connecting mode for the contact point T2b side,
which is connected to the reference voltage E2, and the saturation
preventing circuit 53-3 is set to the connecting mode for the
contact point T3b (which is connected to the reference voltage E3
(saturation preventing operation is enabled), there is no
saturation of the individual gain generating circuits 51-2 and 51-3
even when the value of the DC flow rate signal from the sample hold
circuit 4 becomes large, so there is no impact on the DC flow rate
signal to the A/D converting circuit 6.
Medium Flow Rate
[0081] When the flow rate decreases and the flow speed falls below
1.6 m/s (the point t2 shown in FIG. 6), the processing portion 7A
generates a gain switching instruction to the gain G2, which is the
medium gain of the gains G1 G2, and G3, and sends it to the DC
amplifying circuit 5A.
[0082] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1b side and
of the switch SW2 to the contact point S2a side, sends a saturation
preventing operation disabling setting instruction to the
saturation preventing circuit 53-2 for the connecting mode for the
contact point T2a side, and sends a saturation preventing operation
enabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3b side.
[0083] As a result, as illustrated in FIG. 8, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2a side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3b side.
[0084] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-2 from the sample hold
circuit 4 through the saturation preventing circuit 53-2, the gain
G2 produced by the individual gain generating circuit 51-2 is
applied to the DC flow rate signal, and the DC flow rate signal to
which the gain G2 has been applied is outputted to the A/D
converting circuit 6 of the next stage through the switches SW1 and
SW2 of the gain selecting circuit 52.
[0085] In this case the individual gain generating circuit 51-2 is
selected as the used gain generating circuit, so the output from
the individual gain generating circuit 51-2 is outputted to the A/D
converting circuit 6 in the next stage, but because the saturation
preventing circuit 53-3 is set to the connecting mode for the
contact point T3b which is connected to the reference voltage E3
(saturation preventing operation is enabled), there is no
saturation of the individual gain generating circuits 51-2 and 51-3
even when the value of the DC flow rate signal from the sample hold
circuit 4 becomes large, so there is no impact on the DC flow rate
signal to the A/D converting circuit 6.
[0086] Additionally, the reference voltage E2 that is equal to the
voltage level of the DC flow rate signal that was inputted
immediately previously into the individual gain generating circuit
51-1 is already inputted into the individual gain generating
circuit 51-2 that is selected as the used gain generating circuit,
so the DC flow rate signal is inputted from the sample hold circuit
4 from the state wherein the reference voltage E2 is already
inputted, and thus there is no delay in the amplification process
in the individual gain generating circuit 51-2, so no switching
error is produced.
Small Flow Rate
[0087] When the flow rate further decreases and the flow speed
falls below 0.22 m/s (the point t1 shown in FIG. 6), the processing
portion 7A generates a gain switching instruction to the gain G3,
which is the large gain of the gains G1, G2, and G3, and sends it
to the DC amplifying circuit 5A.
[0088] In this case, the processing portion 7A sends a gain
selecting instruction to the gain selecting circuit 52 for the
connecting mode of the switch SW1 to the contact point S1b side and
of the switch SW2 to the contact point S2b side, sends a saturation
preventing operation disabling setting instruction to the
saturation preventing circuit 53-2 for the connecting mode for the
contact point T2a side, and sends a saturation preventing operation
disabling setting instruction to the saturation preventing circuit
53-3 for the connecting mode for the contact point T3a side.
[0089] As a result, as illustrated in FIG. 7, in the gain selecting
circuit 52 the switch SW1 is set to the connecting mode for the
contact point S1b side, the switch SW2 is set to be connecting mode
for the S2b side, the saturation preventing circuit 53-2 is set to
be connecting mode for the contact point T2a side, and the
saturation preventing circuit 53-3 is set to be connecting mode for
the contact point T3a side.
[0090] As a result, a DC flow rate signal is inputted into the
individual gain generating circuit 51-3 from the sample hold
circuit 4, the gain (B produced by the individual gain generating
circuit 51-3 is applied to the DC flow rate signal, and the DC flow
rate signal to which the gain G3 has been applied is outputted to
the A/D converting circuit 6 of the next stage through the switch
SW2 of the gain selecting circuit 52.
[0091] While in this case the DC flow rate signal is inputted also
into the individual gain generating circuits 51-1 and 51-2 from the
sample hold circuit 4, in the gain selecting circuit 52 the switch
SW1 is set to the connecting mode for the contact point S1b side,
and the switch SW2 is set to be connecting mode for the contact
point S2b side, so the DC flow rate signals that are amplified by
the individual gain generating circuits 51-1 and 51-2 do not pass
through the gain selecting circuit 52, so are not outputted to the
AD converting circuit 6 of the next stage.
[0092] Additionally, in this case the individual gain generating
circuit 51-3 is selected as the used gain generating circuit, so
the output from the individual gain generating circuit 51-3 is
outputted to the A/D converting circuit 6 in the next stage, but
because the DC flow rate signals from the sample hold circuit 4
that are inputted into the individual gain generating circuits 51-1
and 51-2 are small, there is no saturation in the individual gain
generating circuits 51-1 and 51-2, and there is no effect on the DC
flow rate signal to the A/D converting circuit 6.
[0093] Additionally, the reference voltage E3 that is equal to the
voltage level of the DC flow rate signal that was inputted
immediately previously into the individual gain generating circuit
51-2 is already inputted into the individual gain generating
circuit 51-3 that will be selected as the used gain generating
circuit, so the DC flow rate signal is inputted from the sample
hold circuit 4 from the state wherein the reference voltage E3 is
already inputted, and thus there is no delay in the amplification
process in the individual gain generating circuit 51-3, so no
switching error is produced.
[0094] As is understood from the explanation above, in the
electromagnetic flow meter according to the present example the
gain switching is performed by the DC amplifying circuit 5A instead
of by the AC amplifying circuit 3. Additionally, the control of the
enabling/disabling of the saturation preventing operations of the
saturation preventing circuits 53-2 and 53-3, and the selecting
operation for the gain selecting circuit 52, by the DC amplifying
circuit 5A makes it possible to prevent the occurrence of
saturation in the individual gain generating circuits that generate
higher gains than the gain generated by the used gain generating
circuit. Doing so causes constant amplification by the AC
amplifying circuit 3 and prevents the occurrence of saturation in
both the AC amplifying circuit 3 and the DC amplifying circuit 5A,
thereby enabling an increase in the accuracy of the measured flow
rate.
[0095] Note that while in the example set forth above, the number
of switchable gains was 3, the number of switchable gains may
instead be 2, or the number of gains may be further increased. In
this case, first through Nth (where N.gtoreq.2) individual gain
generating circuits 51, wherein the gains thereof are established
so as to be sequentially increasing values are provided, and
respective individual saturation preventing circuits 53 are
provided as the previous stages for all of the individual gain
generating circuits 51 with the exception of the first individual
gain generating circuit 51 with the smallest gain. Additionally,
when the selecting operation by the gain selecting circuit 52 is
controlled by the processing portion 7, the individual gain
generating circuit 51 with the output thereof selected by the gain
selecting circuit 52 is set as the used gain generating circuit,
and the saturation preventing operations of the saturation
preventing circuits 53 that are connected to the individual gain
generating circuits 51 that generate gains equal to or less than
the gain generated by the used gain generating circuit are set to
disabled, and the saturation preventing operations of the other
saturation preventing circuits 53 are set to enabled.
[0096] Note that in the examples set forth above, there is no
individual gain generating circuit that generates a gain that is
less than that of the individual gain generating circuit 51-1, and
thus the individual gain generating circuit 51-1 does become
saturated. Because of this, in the examples set forth above, no
saturation preventing circuit 53 is provided for the individual
gain generating circuit 51-1. However, in a modified example, a
saturation preventing circuit 53 may be provided in a stage
previous to the individual gain generating circuit 51-1, in the
same manner as for the individual gain generating circuits 51-2 and
51-3. In the examples set forth above, the lack of provision of the
saturation preventing circuit 53 for the individual gain generating
circuit 51-1 enables the circuit to be made simpler and less
expensively to that degree.
[0097] The electromagnetic flow meter according to the present
invention can be used in a variety of fields, such as process
control, as a flow meter for measuring the flow rate of a fluid
flowing in a pipe.
* * * * *